China’s Three Gorges Dam is famed for its size — and its reservoir may be large enough to change regional weather patterns. The large body of water has altered temperatures, wind patterns and rainfall rates in both the local Three Gorges Dam region and in the upstream Sichuan Basin, a new study reports — changes that could affect the dam’s ultimate ability to control floods.

At 663 kilometers long and with a capacity of nearly 40 billion cubic meters of water, the Three Gorges Dam’s reservoir is one of the largest in the world (see Geotimes, August 2006). In June 2003, officials raised the reservoir’s water level from 66 to 135 meters — roughly three-fourths of the full depth (175 meters) that it will reach once the project is completed in 2009.

That volume of water has a significant effect on the climate, scientists say, creating a potential evaporating surface that can cool the lower atmosphere and thereby alter both wind and precipitation patterns. Lowering temperatures over the lake means that the air will not rise through the atmospheric column, and without that convection, wind also decreases, says Norman Miller, a climatologist at the Lawrence Berkeley National Laboratory in California. Although the added moisture would suggest a potential increase in rainfall over the reservoir, the sinking air in the atmospheric column actually pushes the cloudy, moist air over the reservoir away, diverting it from the Three Gorges Dam region, Miller says.

Previous studies conducting simulations of the reservoir’s effect on climate had suggested that its impact on temperature and winds would be largely local and could result in no net change in precipitation in the immediate dam area. No studies, however, had yet been conducted that compared rainfall data across the region from the years before and since the reservoir was filled.

To assess how the region’s climate had changed, Liguang Wu, a climatologist at NASA’s Goddard Space Flight Center in Greenbelt, Md., and his team analyzed precipitation rates and land surface temperatures collected by NASA’s Tropical Rainfall Measuring Mission and Moderate Resolution Imaging Spectroradiometer. They looked at two periods, from January 1998 to January 2003 and from January 2004 to January 2006.

The team found that while the average rainfall over the immediate dam area in Yichang had decreased since the reservoir was filled in 2003, it was raining more elsewhere, particularly in the Sichuan Basin, a region ringed by the Daba and Qinling mountain ranges several hundred kilometers to the northwest. In that region, precipitation showed an increase of 1 millimeter per month during the autumn rainy seasons of 2003 and 2005, compared with pre-2003 seasons — a change that is very likely due to the Three Gorges Dam’s construction, Wu says. Daytime temperatures also decreased by about 0.7 degrees Celsius in the same region.

The data suggest that the reservoir is having a regional, rather than local, effect on climate, the team says. This effect is likely to increase once the dam is fully filled, the team reported in the July 7 Geophysical Research Letters.

Miller, who conducted an earlier, smaller-scale simulation of the reservoir’s effect on weather patterns, says that Wu’s team’s results do suggest that significant shifts in climate patterns exist in the region. However, he says, it is difficult to know to what extent those shifts are due solely to the reservoir without also taking into account natural variations in weather — such as an increase in rainfall associated with a 2003 El Niño/Southern Oscillation effect.

The dam’s effect on the climate, however, is “more than likely real,” Miller says. Furthermore, he adds, increased precipitation upstream, when combined with large-scale deforestation in the region, may present a significant problem for the Three Gorges Dam.

“There will be a lot of fast flow and sedimentation,” he says. The dam “may not be able to prevent the larger flooding that it’s intended for.”